This invention relates to chain transfer agents useful in free radical polymerization. Specifically, the present invention is directed toward addition-fragmentation, phosphorus-containing chain transfer agents. These agents are conveniently synthesized by the Perkow reaction.
Chain transfer agents are moieties that react with a growing polymer radical, causing the growing chain to terminate while creating a new reactive species capable of initiating polymerization. Chain transfer agents are useful for controlling the molecular weights of polymers, for reducing gelation when polymerizations and copolymerizations involving diene monomers are conducted, and for preparing polymers and copolymers with useful chemical functionality at their chain ends. The latter application requires that the chain transfer agent be appropriately substituted with the desired chemical functionality. Molecules with multiple chain transfer functionalities can be used for the synthesis of complicated polymer architectures such as block copolymers, graft copolymers, and polymers with star structures.
Most chain transfer agents contain carbon-halogen, sulfur-hydrogen, silicon-hydrogen, or sulfur-sulfur bonds. Except for those that have sulfur-sulfur bonds, these agents cannot be designed to impart useful chemical functionality at both ends of a polymer chain. The most widely used chain transfer agents contain sulfur-hydrogen bonds and are known as mercaptans. The lower molecular weight agents, such as butyl mercaptan, have the disadvantage of being very odiferous. The higher molecular weight agents have the disadvantage of being expensive.
Another disadvantage of the chain transfer agents of the prior art is that their reactivities toward growing polymer radicals are usually considerably different than the reactivities of the monomers involved in the polymerizations. This makes it difficult to obtain uniform products.
A relatively new class of chain transfer agents for free radical polymerization and copolymerization reactions include xe2x80x9caddition-fragmentationxe2x80x9d reagents that add to a growing polymer and the resulting adduct fragments to form a stable polymer molecule and a new free radical that is able to initiate the polymerization of a new polymer molecule. The addition-fragmentation polymerization of styrene in the presence of xcex1-benzyloxystyrene, for example, is shown in FIG. 1.
As another example, many addition-fragmentation reagents cause polymerization reactions to yield macromonomers, which are polymers that contain at their end a group that is capable of polymerizing with other monomers. An example of this is the polymerization of styrene in the presence of the dimer of methyl methacrylate. This occurs as shown in FIG. 2, yielding a product with a reactive double bond at its chain end. Copolymerization of this macromonomer with an acrylate ester, for example, can yield an acrylate polymer with a polystyrene graft, as also shown in FIG. 2. The macromonomer can also behave as a macrotransfer agent and yield a block copolymer.
Many prior art addition/fragmentation chain transfer agents, however, contain allylic hydrogens. These substituents cause the reagents to be sensitive to oxidation during storage. Also, allylic hydrogens can participate in chain transfer reactions that compete with the addition-fragmentation capability of the reagents.
Another drawback to many of the addition-fragmentation chain transfer agents known heretofore in the art is the synthesis of the compounds. That is, many prior art chain transfer agents can only be prepared by expensive, tedious, and dangerous procedures. Indeed, some compounds can only be prepared by using organomercury compounds. For example, xcex1-benzyloxystyrene, which has been used as a chain transfer agent, is very difficult to synthesize inasmuch as known synthesis techniques require an organomercury catalyst that leads to health and disposal problems.
Thus, there is a need for improved chain transfer agents, especially addition-fragmentation chain transfer agents, that do not suffer from many of the drawbacks associated with chain transfer agents known heretofore in the art.
It is therefore an object of the present invention to provide a new class of chain transfer agents that are useful for controlling the molecular weights, end functionality, and structures of polymers.
It is another object of the present invention to provide a method for chain transfer during free radical polymerization using novel reagents.
It is a further object of the present invention to provide a new class of polymers with phosphorus-containing end groups.
It is another object of the present invention to provide polymers with improved homogeneity and a method for preparing the same.
It is yet another object of the present invention to provide addition-fragmentation reagents that have little or no noxious odor.
It is still another object of the present invention to provide chain transfer agents for free radical polymerization that are easy and economical to synthesize and versatile in their applications.
It is a further object of the present invention is to provide a new class of chain transfer agents whose reactivities can be tailored to be similar to those of the monomers being employed during polymerization; this will advantageously result in the monomer and chain transfer agent being consumed together at the same or similar rates.
It is another object of the present invention to provide a new class of chain transfer agents that are polymeric in nature and thereby are useful in the synthesis of polymers and copolymers having a wide range of molecular architectures.
At least one or more of the foregoing objects, together with the advantages thereof over the known art relating to chain transfer agents, which shall become apparent from the specification that follows, are accomplished by the invention as hereinafter described and claimed.
In general the present invention provides a method for making a polymer comprising the step of synthesizing a polymer by employing a chain transfer agent having the formula (II) 
where X and Y are independently selected from oxygen, sulfur, and fully-substituted nitrogen where R1 and R2 are independently selected from hydrogen, halogen, or an organic group, with the proviso that at least one of R1 or R2 be hydrogen or halogen, R3 is an organic group, R4 and R5 are independently selected organic groups.
The present invention also includes a method for making a polymer comprising the step of synthesizing a polymer by employing a chain transfer agent having the formula (I) 
where Y is selected from oxygen, sulfur, and fully-substituted nitrogen, X is selected from oxygen, sulfur, and fully-substituted nitrogen, R1, R2 and R3 are independently selected from moieties that will allow the xcex2 carbon to have a high reactivity with a free radical, and R4 and R5 are independently selected moieties.
The present invention further includes a polymerization process comprising the steps of initiating a free radical polymerization, and effecting chain transfer with a reagent selected from the group including vinyl esters of a phosphorus containing acid, vinyl thioesters of a phosphorus acid, vinyl amides of a phosphorus acid, vinyl esters of a thiophosphorus acid, vinyl thio esters of a thiophosphorus acid, and vinyl amides of a thiophosphorus acid.
The present invention still further includes a method of controlling the molecular weight of polymers resulting from a free radical polymerization including the step of carrying out a free radical polymerization in the presence of a chain transfer agent having the formula (II) 
where X and Y are independently selected from oxygen, sulfur, and fully-substituted nitrogen where R1 and R2 are independently selected from hydrogen, halogen, or an organic group, with the proviso that at least one of R1 or R2 be hydrogen or halogen, R3 is an organic group, R4 and R5 are independently selected organic groups.
The present invention also includes a chain transfer agent having the formula (II) 
where X and Y are independently selected from oxygen, sulfur, and fully-substituted nitrogen where R1 and R2 are independently selected from hydrogen, halogen, or an organic group, with the proviso that at least one of R1 or R2 be hydrogen or halogen, R3 is an organic group, R4 and R5 are independently selected organic groups.